Images are typically recorded and stored as contone images in which each image element has a color tone value. For example, consider a digitally stored color image—each image element will typically have three corresponding values setting tone, among 256 gradations, for example, for each of the primary colors.
Many printing processes, however, cannot render an arbitrary color tone value at each addressable location or pixel. Flexographic, xerographic, inkjet, and offset printing processes are basically binary procedures in which color or no color is printed at each pixel. For example, at each addressable point on a piece of paper, a laser printer can generally either lay down a dot of black or colored toner or leave the spot blank, i.e., white. However, some newer devices have a limited ability to deposit intermediate quantities of colorant, such as toner or ink, or have multidensity colorants.
Digital halftoning involves conversion of the contone image to a binary, or halftone representation. Color tone values of the contone image elements become binary dot patterns that, when averaged, appear to the observer as the desired color tone value. The greater the coverage provided by the dot pattern, the darker the color tone value.
Further, in most print systems, it is necessary to also convert the input contone image, in some color space, to a colorant space of the target device. Many times images are stored such that the pixel level data are in terms of levels of red, green, and blue (RGB). This is most convenient when rendering on common display devices. In contrast, standard printing systems are usually based on a four color pallet of cyan, magenta, yellow, and black (CMYK). The conversion is typically performed using a look-up table (LUT) that maps pixel data comprising red, green and blue levels to pixel data comprising cyan, magenta, yellow, and black levels. These look-up tables are defined by first printing combinations of cyan, magenta, yellow, and black colorants, measuring the resulting colors, and inverting the resulting color table.
To increase image smoothness and color accuracy, it is increasingly common in ink jet printing, for example, to supply more than the standard four colorants (CMYK). In particular, it is common to provide light versions of the cyan and magenta colorants. There light versions can be made with dilute solutions of the colorants. The darker colorants are needed to make fully saturated colors without overloading the paper. However, these heavy inks can make very visible dots on the page, which are especially noticeable in the lighter tones. Therefore, if the lighter inks can be used in the lighter tonal range, the halftone pattern will be less visible. In other examples, additional colorants such as orange and green are added to the device's pallet to improve color rendition.
Increasing the number of colorants results in a more complex conversion to the target device color space. The most straightforward generalization would be to use a 3-to-6 color conversion LUT. However, creating such a table becomes very difficult since it must be created by choosing an inverse from a 6-to-3 table of measurements. This table is very large, and there are many degrees of freedom.
One limitation that constrains the available degrees of freedom in practice concerns the fact that certain ink/media properties or hardware restrictions may require a printing process to limit the allowable maximum total amount of combined colorant, called the total-ink, to a value less than the sum of the maxima of each colorant. Therefore, some mechanism is required to modify the rendering process to enforce these constraints.
A practical example of the application of a total ink reduction is ink-jet printing where on some substrates and when using certain inks, the ink will not stick to the media anymore and cause bleeding of ink when too much ink is used. Another example is in the context of laser jet printing. Although a laser jet can print the maximum amount of ink without visual problems on the print-out, the age of the drum is drastically reduced and may even cause damage when using too much ink for an extended period over large areas. So many laser jet engine manufacturers require the total ink to stay below a certain tolerance level (typically 270%).
Techniques exist for addressing this problem. Some of the most common techniques for controlling total ink (also called total area coverage) are UCR/GCR (under cover removal and grey component replacement). These techniques are applied in many graphic arts related products, such as postscript raster-image processors. They address the way the amount of black colorant is calculated. By calculating first the equivalent neutral density of the desired color, then reducing the amounts cyan, magenta, and yellow colorants appropriately and replacing this neutral component by an equivalent amount of black colorant, in principle the same color is obtained but with a lower total amount of colorant, since the black colorant replaces three colorants.
Other techniques operate by imposing hard limits on the total ink that is applied to a given area. In one such example, after the half-toning of the image data, the half-tone data are analyzed to determine whether the total ink density is higher than a predetermined limit value within a given pixel matrix area. When the limit is exceeded, the ink density is reduced by determining a reduction coefficient that is applied to the quantity of ink applied for each of the chromatic colors. This yields corrected color quantities that are actually applied to the paper.